It really depends on the flavor. I notice coils do not come clean easy with real sweet or chocolate flavors. Coffee as well makes them black fast. I rewick about every 25ml on sweet fluids and sometimes as long as 50ml for light flavor. I use the rule of thumb that if coils will not come clean or are getting really difficult to clean I put in new ones. Sometimes when I am bored i will grab my drill and just make fused claptons or something different because or boredom but it doesn't mean my coils really needed changed.